1. Field of Invention
The present invention relates to synthetic aperture radar; and more particularly to a method and system for sharpening the impulse response of the return signals of a synthetic aperture radar.
2. Description of Related Art
Synthetic aperture ground mapping utilizes a system that generates pulses continuously at a frequency constant enough to be coherent for a time interval during which an airborne radar platform travels a predetermine distance. All echoes returned during this distance are processed as though it were a single antenna having an aperture as long as the distance traveled by the platform during each particular time interval. The scanning in range, of course, corresponds to the speed of light, and the scanning in azimuth corresponds to the speed of the airborne platform. The location in azimuth is determined by the Doppler Shift and the location in range is based upon the time of return.
Resolution is typically defined as the ability to distinguish two closely spaced targets. However, with respect to mapping, resolution is also referred to in terms of the two dimensions; that is, the ground area dimensions in range and azimuth that are distinguishable by the radar system. Although, in synthetic aperture ground mapping, the resolution capability of the radar is extended beyond the limits of frequency and antenna aperture size, there are several factors which tend to spread the target spectrum resulting in resolution loss both in range and azimuth. For example, in the synthetic array concept, the received signals are phase shifted and summed during each of the predetermined time intervals to provide a constant phase front at the desired pointing angle so that the return signals are effectively added in phase. However, in the time it takes to form the synthetic aperture, the aspect angle to the target changes slightly which results in the targets doppler changing and resulting in a spread of the target spectrum.
Additionally, since the amplitude return from a point target is relatively flat as it enters the receiver, the received signal is weighted to reduce the side lobe levels. However, in improving the side lobe levels, there is an accompanying loss of resolution because of the increase in spectral width of the response. This loss of resolution is particularly noticeable with respect to isolated point targets, and also boundaries where there is a departure in elevation, such as buildings or other abrupt changes.
In light of the foregoing, there is a need to provide a synthetic aperture radar where the side lobes of the return energy are effectively reduced, and yet impulse responses corresponds to the edge contrast of boundaries and isolated point targets are enhanced.